This invention relates generally to superconductor circuits and devices, and in particular the invention relates to Josephson Junction Devices used in superconductive integrated circuits.
Chapter 4 of Superconductive Devices and Circuits, 2nd Ed. (by T. Van Duzer and C. W. Turner) Prentice Hall, 1999, describes superconducting Josephson devices. When a direct current (DC) voltage (V) is applied to two superconductors separated by a very thin insulating layer or a conducting material, the frequency of the alternating current (AC) voltage developed between the superconductors is equal to 2 eV/h where e is the electric charge and h is Planck""s constant. Current flows through the insulator by tunneling or through the conducting layer by xe2x80x9cproximity coupling.xe2x80x9d This effect is called the Josephson effect. Its applications include high speed switching of logic circuits and memory cells (well under 100 ps), parametric amplifiers operating up to at least 300 gigahertz (GHz), and maintenance of the U.S. legal volt at the National Institute of Standards and Technology. The Josephson devices used in integrated circuits are required to be controllable and repeatable. Also, they need to have a large product of critical current (Ic) and normal resistance (Rn), or IcRn, to be useful for high speed operation. Pulse widths are approximately inversely proportional to IcRn; and reduced pulse width allows operation at higher frequencies.
Most existing applications of superconductor electronics currently use niobium for the interconnections and resistively shunted Nb/AlOx/Nb Josephson tunnel junctions as the active devices. Niobium is preferred over other materials for interconnections because of its chemical and physical stability and because a key parameter called the London penetration depth, has an advantageous value compared with, for example, niobium nitride, or the so called xe2x80x9chigh-temperature superconductors.xe2x80x9d
FIG. 1 is a plan view of a conventional Nb/AlOx/Nb Josephson junction device including a tunnel junction 10 between a niobium conductor 12 and overlying niobium conductor 14 with a shunt resistor 16 connected between conductor 12 and conductor 14 in parallel with tunnel junction 10. Resistor 16 uses considerable space in an integrated circuit thus reducing circuit density and introducing parasitic inductance that degrades circuit performance and introduces complexities in circuit design.
The present invention provides a Josephson junction device which can be readily employed in single flux quantum logic circuits and superconducting quantum interference devices (SQUIDS).
In accordance with the invention, a Josephson junction device is provided which has internal shunt resistance, thus obviating an external shunt resistor. The device requires less surface space in an integrated circuit and reduces parasitic inductances. The device has applications in which current-voltage characteristics must be nonhysteretic and for which a large value of the product of the critical current and the normal state resistance (IcRn) is desired.
Briefly, in one embodiment the Josephson junction device comprises five layers, including two outer layers of Nb with layers of NbyTil-yN, TaxN, and NbyTil-yN (where 0.2 less than y less than 1 and 0.2 less than x less than 2) therebetween. Here the NbyTil-yN is illustrative of a more general superconductor with Tc greater than 9xc2x0 K and penetration depth greater than that of Nb, and TaxN is illustrative of a conducting material with a resistivity between 200 xcexcxcexa9-cm and 1 xcexa9-cm. One layer of Nb can be supported on an insulating layer such as silicon dioxide over a superconductive ground plane on a silicon substrate. In this embodiment, all layers are thin films with film thicknesses in the range of 100-200 nanometers, except for the barrier material (typically, TaxN with 0.2 less than x less than 2), which can be in the range of 10-500 nanometers.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing.